The present invention pertains to ceramic items and methods for preparing ceramic items and more particularly pertains to ceramic articles, tools, and methods for preparing ceramic articles and for sintering. Zirconia (ZrO.sub.2) is a ceramic material which, in its tetragonal crystal structure, is strong and tough but has the shortcoming that it is relatively soft. Zirconia can also exist in the cubic and monoclinic crystallographic structures, both of which are harder but more brittle than the tetragonal structure. Cubic phase zirconia requires high temperatures for its formation. Monoclinic zirconia, on the other hand, is unsuitable for common fabrication processes because of the enormous thermal stress generated during sintering which causes crumbling of the ceramic. For a number of applications, such as cutting tools, it is desired that the tool materials possess both hardness and toughness.
One of the ways of improving the mechanical properties of crystalline materials is dispersion hardening where distinct second phases are dispersed in a base matrix. To be effective, these second phases should be harder and stronger than the matrix, adding strength through both their reinforcing action and by the additional barriers presented to crack propagation. Such a composite material can provide the optimum combination of strength and toughness desirable for engineering applications. However, the choice of the second phase material, its size in the dispersed phase and the uniformity of the dispersion in the base matrix are crucial to the performance of the composite material.
The hard second phase material should be so chosen that there is compatibility between the dispersed phase and the base matrix. Materials which can chemically interact with the base matrix may result in an undesirable reaction product deteriorating the properties of the composite. Mismatch of thermal expansion coefficient between the dispersed phase and the base matrix may generate stresses around the dispersed phase promoting intergranular fracture and thus lowering the toughness of the composite, as observed by Hogg and Swain (C. L. Hogg and M. V. Swain in "Advances in Ceramic, vol. 24A, Science and Technology of Zirconia III, eds. S. Somiya, N. Yamamoto, H. Yanagida, The American Ceramic Society, Ohio, p. 253, 1988). The size and the interparticle spacing of the dispersed phase also play a major role in controlling the final microstructure of the composite. The dispersed second phase can act to pin the grain boundary and prevent excessive grain growth of the base matrix. Accordingly, finer or smaller sized dispersions are expected to be better in checking grain growth at high temperatures, usually needed for obtaining high density composites. For zirconia based systems this is an important issue since tetragonal-to-monoclinic phase transformation in zirconia is strongly dependent on the grain size. Thus the size and the interparticle spacing of the dispersed phase in zirconia systems can have a large effect on the mechanical properties of the composite as well as on the processing conditions such as sintering temperature.
For zirconia based systems, addition of alumina is often practiced for improving mechanical properties. Conventionally this is achieved by mixing of the oxides in dry form prior to compaction. The requirements of this process are: (1) the powder must be deflocculated through mutual dispersion of the different oxides to achieve optimum powder packing;. (2) the two dissimilar oxides should be distributed in such a way as to have individual alumina particles as neighbors of zirconia particles to prevent excessive grain growth during sintering of the zirconia particles resulting in inferior mechanical properties. Obviously, it is very difficult to achieve uniform dispersion on a large scale even with efficient mixing equipment. Lange et al (F. F. Lange, B. I. Davis and I. A. Aksay, J. Am. Ceram. Soc. 66 (6), p.398, 1983) demonstrated that dry powder routes to powder consolidation of zirconia-alumina systems can produce large agglomerates which result in large crack-like voids due to differential sintering.
One alternative to dry mixing of dissimilar powders such as zirconia and alumina is the aqueous dispersion of colloidal suspensions of zirconia and alumina. Aksay et al reported (I. A. Aksay, F. F. Lange and B. I. Davis, J. Am. Ceram. Soc. 66 (10), p. C190, 1983) uniform dispersion of the two phases in an aqueous medium maintained at pH values of 2.0-3.5. Solid composites prepared from this dispersion were reported to have improved flexural strength. However, from a practical point of view, the method of producing well dispersed zirconia-alumina systems from a low pH aqueous medium will need additional processing steps of (1) separation of the solid from the aqueous medium (by filtration, gravitation or centrifugal settling) and (2) subsequent drying, adding cost to the final product. Moreover, low pH slurries pose the additional problem of corrosion of containers, stainless steel spray dryers, plaster molds, etc. as well as contamination of the ceramic from the corrosion products. Thus the need for zirconia based ceramic articles with improved properties which can be obtained by a simple fabrication process exists in the prior art.
It is an object of the invention to provide improved ceramic articles and tools, and improved methods for preparing ceramic articles and sintering; in which a hard monoclinic phase is well dispersed in a continuous base matrix of tetragonal zirconia, with the dispersed second phase being less than 100 nm in size. In the broader aspects of the method for preparing ceramic articles of the invention, there is provided a method for preparing a ceramic article comprising compacting a particulate material, including a primary oxide and a secondary oxide, where the oxide particles are coated with a combination of a smectite clay and a suitable polymeric material, to form the green blank. The primary oxide is ZrO.sub.2. The secondary oxide is selected from the group consisting of MgO, CaO, Y.sub.2 O.sub.3, Sc.sub.2 O.sub.3, rare earth oxides and combinations thereof. The polymeric material can be a water soluble polymer (e.g., polyvinyl alcohol, polyethylene glycol, polyethylene oxide, polystyrene sulfonate, polyacrylamide, polyvinyl pyrrolidone, etc.), a hydrophilic colloid (e.g., gelatin) or a water insoluble latex or dispersion (e.g., polymers and interpolymers of styrene, styrene derivatives, alkyl acrylates or alkylmethacrylates and their derivatives, olefins, acrylonitrile, polyurethane and polyester ionomers) or combinations. The polymeric material is combined with a sol containing a smectite clay, preferably a synthetic smectite which is a hydrous sodium lithium magnesium silicate or fluorosilicate with a layered structure. The smectite clay-polymer combination, henceforth referred to as the "binder", is coated on the oxide particles which are compacted into green blanks and subsequently sintered to form the desired ceramic article. The sintered zirconia alloy ceramic article thus produced has a continuous base matrix of tetragonal zirconia in which a hard monoclinic phase is well dispersed with the dispersed second phase being less than 100 nm in size.